Materials Flashcards
What is the density of a material?
The mass per unit volume.
It measures how compact an object is.
What is Hooke’s Law?
Hooke’s Law states that extension is directly proportional to the force applied, given that the external conditions are kept constant.
F = kx
where
k = spring constant (measure of stiffness) (N/m)
x = extension (m)
How can we see from a graph that an object is following Hooke’s Law?
- On a force-extension graph
- A straight line through the origin with a linear relationship
What is the limit of proportionality?
The point after which Hooke’s Law is not longer obeyed.
What is the elastic limit of a material?
The point after which plastic deformation will occur.
What is the relationship between the limit of proportionality and the elastic limit?
- On a force-extension graph
- Limit of proportionality occurs before the elastic limit
What is tensile stress?
The force applied per unit of cross-sectional area. (measured in Pascals)
Stress = F/A
where
F = force applied (N)
A = cross-sectional area (m2)
What is tensile strain?
The ratio of the extension to the original length (no units).
Strain = x/L
where
x = extension (m)
L = original length (m)
What is elastic strain energy?
When work is done to stretch or compress a material, the energy is stored as elastic strain energy
It can be worked out as the are under the force-extension graph.
Elastic strain energy = 1/2 Fx
What is the breaking stress of a material?
The value of tensile stress at which the material will break apart.
This value changes depending on the conditions of the material.
What does it mean for a material to be plastic?
The material will experience a large amount of extension as load is increased especially beyond the elastic limit.
What does it mean for a material to be brittle?
The material will extend very little, and therefore is likely to fracture at a low extension.
What is the work done in deformation?
- On a force-extension graph
- The area between the loading and unloading curves
- If elastic deformation occurs, this area is the work done heating the rubber (thermal energy)
- In plastic deformation, this area is the work done to permanently deform
What can we say about the loading and unloading lines in both elastic and plastic deformation?
For both, the lines will be parallel. However, for plastic, the unloading line does **not go through the origin.
What does the elastic deformation loading and unloading curve look like?
What is work done used for in elastic and plastic materials?
Elastic: stored as elastic strain energy
Plastic: move atoms apart, dissipated as heat
What is the difference between a force-extension and a stress-strain graph?
- Force-extension describes the behaviour of the object
- Stress-strain describes behaviour of the material
What is the ultimate tensile stress of a material and where is it found relative to the limit of proportionality and the elastic limit?
- The highest point on the stress-strain graph
What does the ultimate tensile stress if the material mean?
The highest stress that that the material can withstand before fracturing.
What is the Young Modulus of a material?
A value which describes the stiffness of a material.
Young Modulus. (Pa) = Tensile Stress (Pa) / Tensile Strain
or
E = FL/AΔL
How can we find the Young Modulus from the stress-strain graph?
Finding the gradient of the straight part of the graph.
